1. Field of the Invention
This invention relates to atmospheric ionization of analytes with metastable atoms and molecules. Metastable atoms and molecules (M*) are excited-state species with long lifetimes. Metastable species are produced in corona or glow electrical discharges. Other methods of producing excited-state species include electron impact, photoionization, and controlled interaction of high energy particles with a reactant species. Collisions between excited-state species and ground-state species can result in ionization of the ground-state species and release of electrons by a process known as Penning ionization, for example:M*+N->N++M+e-  Equation 1
2. Description of Related Art
Regulatory and safety issues related to the use of radioactive materials, such as 63Ni, 241 Am, and 3H, among others, have led to a search for non-radioactive ion sources for analytical instruments, such as ion mobility spectrometers. (See Turner et al. U.S. Pat. No. 6,225,623 entitled “Corona Discharge Ion Source for Analytical Instruments” and Doring U.S. Patent Application Publication No. 2002/0185593 entitled “Ion Mobility Spectrometer with Non-Radioactive Ion Source”.)
Certain available corona discharge ion sources for atmospheric pressure ionization (API) mass spectrometers or ion mobility spectrometers (IMS) or chemical agent monitors (CAM) introduce the analyte (including solvent, air, and other contaminants) into the region containing a discharge needle. This leads to several problems:
1. The presence of oxygen or other contaminants in the air leads to degradation of the electrodes.
2. It can be difficult to maintain the discharge in the presence of contaminants, requiring a high electrical potential or pulsed potentials.
3. A corona discharge in air leads to the formation of species, such as NO2−, NO3−, and related cluster ions. These ions can cause a loss of sensitivity for analyte ions (C. A. Hill and C. L. P. Thomas, Analyst, 2003, 128, pp. 55-60) and can interfere with the detection of NO2− and NO3− produced from analytes containing nitro functional groups, such as nitro explosives or in the case of chloride ion interference with chlorate propellants and rocket motors or phosphate interference with chemical warfare-related compounds.
4. Introducing air and analyte into the discharge region limits the possibilities for controlling the nature of the chemical background to control the ion-formation chemistry.
Taylor et al. U. S. Pat. No. 5,684,300 entitled “Corona Discharge Ionization Source” and Turner et al. U.S. Pat. No. 6,225,623 B1 entitled “Corona Discharge Ion Source for Analytical Instruments” describe corona discharge ion sources, but do not describe a means for separating the region where the discharge occurs from the region where the analyte is introduced. See also Zhao et al. entitled “Liquid Sample Injection Using Atmospheric Pressure Direct Current Glow Discharge Ionization Source,” Anal. Chem, 64, pp.1426-1433, 1992.
Bertrand et al. U.S. Pat. No. 6,124,675 entitled “Metastable Atom Bombardment Source” discloses a metastable atom source operating at reduced pressure for generating ions in a mass spectrometer. The device described requires substantially reduced pressures and does not describe means for using metastable atoms for atmospheric pressure ionization mass spectrometry or ion mobility spectrometers.
Tsuchiya et al. U.S. Pat. No. 4,546,253 entitled “Apparatus for Producing Sample Ions” describes a method for using metastable atoms to produce ions from a sample introduced at the tip of an emitter needle downstream from the corona discharge. This technique requires that the sample be placed on or near an intense electric field emitter needle. See also Otsuka et al. entitled “An Interface for Liquid Chromatograph/Liquid Ionization Mass Spectrometer,” Analytical Sciences, Vol. 4, Oct. 1988. The present invention avoids use of an emitter needle at high electrical potential placed downstream of the corona discharge source. Further, the present invention provides a means of sampling neutral analyte molecules without the restriction of relocating the analyte from the surfaces on which they are attached. For example, cocaine from cash currency, and chemical/biological warfare agents from surfaces of military interest can be sampled directly and in situ without swabbing or solvent washing the surface. Each time sample is relocated, analyte molecules are lost (30 to 100% for trace-level concentrations). Therefore, direct surface sampling is always preferred.